Tetramethylbenzene production



United States Patent TETRAMETHYLBENZENE, PRODUCTION David A. McCaulay, Chicago, Ill., assignor to Standard Oil (Company, Chicago, 11]., a corporation of Indiana No Drawing. Application July 26, 1955,

. Serial No. 524,570

6 Claims. (Cl. 260-668) This invention relates to the production of tetramethylbepzene by the disproportionation of trimethylbenzene.

Very recently the chemical industry has become intensely interested in tetraalkylbenzenes wherein the alkyl group is either methyl or ethyl. These tetraalkylbenzenes may be converted to various alkyl benzoic acids; particularly there is interest in the benzene nucleus containing 4 carbcxylic groups. Most interest is concentrated in the tetraalkylbenzene having the durene configuration, i. e., i,2,4,5- which leaves two unsubstituted ring positions in para orientation.

The tetramethylbenzenes are not particularly plentiful in either coal carbonization liquids or in catalytic reforrnate from conversion of petroleum naphthas. Furthermore, the close-boiling aromatic hydrocarbons containing a total of 10 carbon atoms make the problem of obtaining high purity tetramethylbenzenes exceedingly dilficult and expensive. Catalytic reformate from the conversion of petroleum naphtha does contain quite large amounts of benzenes containing a total of 9 carbon atoms, primarily trimethylbenzenes and ethyltoluenes, It is possible by fractional distillation to obtain a fraction consisting essentially of about 90 mole percent trimethylbenzenes and not more than about 10 mole percent of ethyltoluene; with some loss in yield of trimethylbenzene, it is feasible to reduce the ethyltoluene content to on the order of 5%. It is known that trimethylbenzenes can be converted to tetramethylbenzenes and xylene utilizing various acid-type catalysts. For example, HF-BFs catalyst readily converts trimethylbenzenes to tetramethylbenzenes, but the isodurene isomer is the predominant member and, under some conditions, essentially the only component of the tetramethylbenzene product. Other acid catalysts tend to produce a mixture of tetramethylbenzenes wherein isodurene is predominant.

An object of this invention is the production of tetramethylbenzene, particularly a mixture of tetramethylbenzenes wherein durene is the predominant component. Another object of the invention is a more economical process for disproportionating trimethylbenzenes to a mixture of tetramethylbenzenes. Yet another object isa process for utilizing a mixture of C9 aromatic hydrocarbons containing some ethyltoluene and the remainder trimethylbenzene in high yield to tetraalkylbenzenes. A further object is a process for disproportionating trimethylbenzenes wherein the by-product xylene fraction is of superior economic value. A particular object is an economic disproportionation process for converting trimethylbenzenes to tetramethylbenzenes utilizing liquid HF as the catalyst. Other objects will become apparent in the course of the detailed description.

In the process of this invention, a trimethylbenzene or a mixture of trimethylbenzene isomers is contacted with between 25 and 100 volume percent of liquid HF catalyst; the contacting i carried out at a temperature between about 80 C. and 190 C. for a time sufiicient to produce an appreciable amount of tetramethylbenzene and by-product xylene. In general, the higher the tempera- 2,803,681 Patented Aug. 20, 1957 ture, the shorter the contacting times used. The HF is separated from the product hydrocarbon mixture and the desired tetramethylbenzene fraction is recovered from said mixture.

The hydrocarbon feed to the process may consist en tirely of trimethylbenzene, either a single isomer or a mixture of two or all of the isomers. The isomeric trimethylbenzenes are hemimellitene (l,2,3-trimethylbenzene), pseudo cumene (1,2,4-trimethylbenzene), and mesitylene (1,3,5-trimethylbenzene). Or the hydrocarbon feed may consist of one or more trimethylbenzene isomers and one or more ethyltoluene isomers; as much as 10 mole percent of ethyltoluene may be present in the feed without any appreciable adverse effect on the yield of tetramethylbenzenes. It is preferred to have not more than about 5 mole percent of ethyltoluene present. In addition to the trimethylbenzene and ethyltoluene, the feed may contain a small amount of close-boiling nonaromatic hydrocarbons. The presence of non-aromatic hydrocarbons introduces a complication in the purity of the product and also cracking reactions are introduced at the higher temperatures of operation. Some of the cracked non-aromatic hydrocarbons alkylate the benzene hydrocarbons with a consequent loss of benzene hydrocarbons to undesired by-products. A suitable source of trimethylbenzene feed is the C9 fraction obtained by extractive distillation of a catalytic reformate fraction; this extracted aromatic-rich C9 fraction which contains on the order of not more than 5 volume percent of non-aromatic hydrocarbons is fractionally distilled to eliminate substantially all of the C9 aromatic hydrocarbons which do not boil closely about the trimethylbenzene range. A trimethylbenzene concentrate containing ll) mole percent of ethyltol'ueries and some non-aromatic hydrocarbons is readily obtained. By careful distillation it is possible to reducethe ethyltoluene content of the concentrate to about 5 mole percent. A particularly suitable feed for the process is obtainable from the C9 aromatic hydrocarbon fraction produced by glycolic solvent extraction of catalytic reformate; this fraction contains essentially to no non-aromatic hydrocarbon component; thus it is possible to produce by fractional distillation a concentrate containing between about 5 and 10 mole percent of ethyltoluenes and the remainder essentially only trimethylbenzenes. A source of essentially pure trimethylbenzenes is the product from the disproportionation of xylenes using liquid HF catalyst and particularly liquid HF- BFs catalyst.

The catalyst utilized in the process of this invention is substantially anhydrous hydrofluoric acid or even liquid hydrogen fluoride. In orderto maintain catalyst activity at very high levels, the liquid HF should not contain more than about 2 or 3 Weight percent of water. In order to maintain the water content of the system at a low level, the entire process is carried out under substantially anhydrous conditions, i. e., the feed hydrocarbons themselves must be of low water content in order to avoid introducing deleterious amounts of Water into the system.

It has been found that the trimethylbenzenes disproportionate very readily when utilizing substantially anhydrous liquid HF catalyst. The rate of conversion is so great that it is not necessary to use very large amounts of catalyst. Appreciable conversion can be obtained with rather small amounts of catalyst Furthermore, it is not necessary to use more than volume percent of liquid HF catalyst based on feed hydrocarbons. In general, the catalyst usage will lie between 25 volume and 100 volume percent based on feed hydrocarbons. It is preferred to operate With between about 50 and 80 volume percent of catalyst.

The disproportionation process is carried out at a temperatilre Within the range of about so c. and c The disproportionation reaction approaches an equilibrium condition at which about one-half of the trimethylbenzenes charged have been converted to tetramethylbenzenes and xylene. Thus the time for which the contacting of feed and catalyst must be carried on is dependent upon the degree of conversion of the trimethylbenzenes desired as well as on the temperature of contacting and the amount of catalyst used. The rate of conversion decreases very rapidly after about the 30% conversion point has been reached and it is considered that for normal purposes the process will be car ried out while maintaining the degree of conversion between about and 30% of the trimethylbenzenes charged. At constant conversion, the contacting time is dependent upon the temperature of operation and also on the amount of catalyst used. Broadly, the lower the temperature, the longer the corresponding time at fixed catalyst usage. And the lower the catalyst usage, the longer the corresponding time at a fixed temperature.

'When operating with the preferred catalyst usage of between about 50 and 80 volume percent at the preferred temperature of between about 130 C. and 170 C. and at a trimethylbenzene conversion between about 20 and 30 mole percent, the time of contacting is between about 3 minutes and about 60 minutes, the lower times corresponding to the higher temperatures.

Based on experimental data, the relationship between temperature, catalyst usage and time to reach conversion of about and 44% have been calculated. This relationship is set out in Table I. The flexibility of the process is clearly shown by the information set out in Table I. Thus the throughput for a given piece of equipment may be radically changed by a small change in temperature, or the degree of conversion may be controlled very accurately at a given temperature by changing the amount of catalyst used.

TABLE I Minutes for 25% Minutes for 44% Conversion Conversion 0. HF, Vol. percent HF, Vol. percent on Feed on Feed of the tetramethylbenzene product does not immediately tetramethylbenzene product of this process.

tetracarboxylic acids.

reach the equilibrium composition. The durene isomer appears to be the predominant product of the disproportionation and isodurene appears to be formed by isomerization of durene. By limiting the degree of conversion to between about 20% and 30%, it is possible to produce a tetramethylbenzene product fraction wherein durene is the predominant component. Since durene is the preferred commercial tetramethylbenzene, this is a result most eminently desired.

A further interesting feature of the process is in the composition of the xylenes which are produced in the disproportionation reaction. At low conversions of trimethylbenzene the xylenes produced consist entirely of about equal amounts of the ortho and meta isomers. No para-xylene appears to be formed. However, as the degree of conversion of trimethylbenzene increases, paraxylene appears in the xylene fraction and eventually the equilibrium mixture appears consisting of about 60% of meta, 25% of ortho, and 15% of para. Since high .purity ortho-xylene and meta-xylene may be readily fractionated from a mixture containing only these isomers, it is desirable to control the degree of conversion vof the trimethylbenzene to avoid the formation of paraxylene whose presence complicates this separation. By operation in the preferred region of 20% to 30% conversion a xylene by-product fraction containing essentially no para-xylene is obtainable.

Durene which has a melting point of about 80 C. is very readily separated by fractional crystallization from .the isodurene and prehnitene isomers which melt respectively at 24 C. and 6 C. Thus by a very simple conventional fractional crystallization procedure it is possible to separate essentially pure durene from the Since isodurene and prehnitene have a boiling point difference of about 7 C., high purity prehnitene may be readily produced by fractional distillation of the mother liquor from the fractional crystallization procedure.

When ethyltoluene is present in the feed to the process, ethyltrimethylbenzenes are formed in addition to the tetramethylbenzenes. The ethyltrimethylbenzenes can be separated by fractional distillation from the tetramethylbenzenes. When a raw material is desired for the production of polycarboxylic aromatic acids, the tetramethylbenzenes and the ethyltrimethylbenzenes can be charged to the oxidation process to produce a mixture of The presence and location of the ethyl group will not aflect the composition of the tetracarboxylic acids. The various isomers of the acids may be separated by well known techniques. The presence of small amounts of the ethyltoluene will have no deleterious effect on the process.

The results obtainable by the process of the invention are set out in many illustrative examples. The tests were carried out in a one-liter Hastelloy autoclave provided with a motor-driven stirrer. In the experimental procedure, the feed hydrocarbon was charged to the autoclave and the whole heated to about 30 C. above the desired reaction temperature. The liquid HF catalyst (commercial anhydrous hydrofluoric acid containing 99.5% HP) was then charged to the autoclave; the desired reaction temperature was thereby reached within a few seconds. The mixture was stirred for the desired time and at the end of this time the entire mixture was withdrawn into a polyethylene flask immersed in a Dry-Ice acetone bath. About one volume of cold water per volume of liquid HF charged was added to the flask.

The upper layer of hydrocarbons was separated from the fractions were analyzed by infrared absorption techniques 'for individual isomer content.

The pseudo cumene utilized was 99.4% pure. The

other hydrocarbons were purchased from the Eastman Kodak Company and were white label grade.

Example A In this example, three tests were carried out utilizing a mixture or trimethylbenzenes as the feed hydrocarbon. In Test No. 1, some indane impurity was also present. The results of this example are set out in Table 11. Test No. 3 shows that at 100. C. and 120 minutes time only 10% of the feed was converted. Test No. 2 shows that at 165 C. and only minutes time 28% of the feed was converted. Test No. 1 shows that at 135 C. and a time of 240 minutes about one-half of the trimethylbenzenes were converted. This test shows that some reaction took place between indanes and the trimethylbenzenes.

Tests 9 through 11 were carried out at 165 C. and 100 volume percent of liquid HF catalyst and show the efiect of time on the process. At three minutes time (Test No. 11) 30% of the pseudo cumene was converted; at 10. minutes time (Test No. 10) 36% was converted, and Test No. 9 indicates that 240 minutes is too long since only 50% was converted despite this very much longer contacting time. The product tetramethylbenzene composition shows the importance of degree of conversion on the amount of durene present. At this very high temperature, even at 3 minutes time, the tetramethylbenzene fraction contained only 50 mole percent of durene. In 10 minutes, the durene and isodurene amounts had reversed and isodurene was predominant. Test No. 9 indicates that the equilibrium composition is about: Isodurene, 50%, durene, 45%, and prehnitene,

TABLE II 5%, since it is the same as that obtained in Test No. 10.

The composition of the product trimethylbenzene frac- T tN 1 2 3 tion is of interest in that the data show that pseudo es 0 cumene is not only disproportionated but is also isom- F d mole ement, erized. Test No. 6 shows that 15% of the unconverted r'seudo umene' 11 65 e5 pseudo cumene had been isomerized to mesitylene and i 0 30 30 hemimellitene. Runs 8 through 11 indicate that an equi- Hemunell1tene. 82 5 5 dane 7 llbrlum condition is reached wherein the product tn- HF1 Fee 100 100 methylbenzene fraction consists of pseudo cumene, 65%, Temperature, C 135 165 100 Product Recoyery, Wt. percent. 9s 9s 97 mesrtylene, and hemimellitene, 5%. gf g ffj 3 0 0 Tests 6, 8, and 9 show the product xylene compositions. X leneIIIIIII 21 14 5 In Test 6 where only 18% of trimethylbenzene charged @Qgfifgfififigg is E g was converted the product xylenes consisted of equal molar Higher boiling 9 0 0 30 amounts of ortho-xylene and meta-xylene. In Test No.

1 Mixture of pentamethylbenzene and methylindanes.

Example B In this example, a series of tests were carried out utilizing pseudo cumene (99.4% purity) as the sole feed component. The results of this series of tests are set out in Table III. Tests 4 through 7 show the efiect of HF catalyst usage on the degree of conversion of the pseudo cumene. Thus at constant temperature of 100 C. and constant time of 60 minutes, changing the HF usage from 25% to 60% resulted in an increase in conversion from 6% to 15 A further increase to 100 volume percent of catalyst resulted in a conversion of 18%. Under these conditions, only 28% was converted when the catalyst usage was increased to 212%. These four runs shown that there is no real economic incentive to operating at above 100 volume percent of liquid HF catalyst.

Test No. 6 shows that at 18% conversion, durene formed 55 mole percent of the tetramethylbenzene fraction, whereas Test No. 7 shows that durene had decreased to 49% of the tetramethylbenzene product when the conversion had increased to 28%.

Example C In this example, the extent of possible conversion of trimethylbenzenes and the equilibrium distribution of tetramethylbenzene isomers was studied. The tests were carried out at 100 volume percent of liquid HF based on feed and a temperature of 165 C. In Test 13, pseudo cumene alone was contacted for 240 minutes. In Test No. 14, a mixture consisting of of isodurene and of mixed xylenes was contacted for 5.0 minutes. In Test No. 15, about an equal molar mixture of durene, pseudo cumene and xylene was contacted for 8 minutes. In Test No. 16, a mixture of durene, 25%, pseudo cumene, 50%, and xylene 25%, was contacted for 8 minutes. The results of these tests are set out in Table IV.

TABLE III.PSEUDO CUMENE Test No 4 5 6 7 8 9 10 11 12 HF, Vol. perceglt on Feed 25 212 100 100 100 100 100 Temperature, C 100 100 100 100 165 165 196 Time, Minutes; .c 60 60 60 60 240 240 10 3 40 Product, Recovery, Wt. percent 98 98 97 98 96 95 96 Product Distribution, Mole percent:

Toluene 0 0 0 0 0 2 0 Xylene- 3 7 9 14 19 22 18 Trlmethylbenzene. 94 85 82 72 62 51 64 Tetramethylbenzene 3 8 9 14 19 25 18 Pentamethylbenzene 0 0 0 O O 0 0 Product Trimethylbenzene, Mole percent:

Pseudo Cumene- 85 65 65 65 Mesitylene 12 35 3O 30 Hemimellitene 3 5 5 5 Product 'Ietramethylbenzene, Mole Durene 55 49 48 45 45 Isodurene- 42 47 48 5O 50 Prehnltene 3 4 4 5 5 Product Xylene, Mole percent:

Ortho- 50 40 25 Meta-. 50 50 Para- O l0 1 Tar and gas also formed.

Test No. 16 shows that the presence of durene in the feed does not have any efiect on the distribution of the tetramethylbenzene product. At' this short time of contacting, it appears that no prehnitene was formed. However, the durene and isodurene ratios closely approached those obtained in Test No. 13 when pseudo cumene alone was the feed.

Test No. 16 also indicates that the presence of xylene and durene in the feed had no effect on the isomerization of the pseudo cumene to the mesitylene and hemimellitene isomers, as evidenced by the distribution in Test No. 13. Test No. 14, when compared with Test No. 13, indicates that isodurene and xylene interact under these conditions to produce an equilibrium mixture of xylene, trimethylbenzene, tetramethylbenzene and some pentamethylbenzene. In both Tests 13 and 14, about one-half of the hydrocarbon product consists of trimethylbenzene, even though in one test only trimethylbenzene was charged and in the other no trimethylbenzene was charged. containing xylene, trimethylbenzene and tetramethylbenzene will adjust itself to what appears to be the equilibrium distribution of the three polyalkylbenzene materials. Thus in Test No. 5, the product contained very close to the same distribution as that in Tests 13 and 14 where entirely diflerent feeds were charged. In Test No. 16, the supposition that the product distribution had an equilibrium content of about 25 mole percent of xylene,

50 mole percent of trimethylbenzene and 25 mole percent r of tetramethylbenzene was tested. Within the error of 6 the determination, the product hydrocarbons had the same polyalkylbenzene distribution as the feed. The evidence that, under these conditions, reactions took place is present in the isomerization of pseudo cumene to a mixture of trimethylbenzenes and of the durene to a mixture of durene and isodurene. Thus, based on the data presented in Table IV, it appears that any process for the production of tetramethylbenzene by the HF- catalyzed conversion of trimethylbenzenes must consider itself limited to about 50% conversion per pass.

TABLE IV [1113, 100 vol. percent on feed; temperature, 165 0.]

Test N 13 14 15 16 Feed, Mole percent:

Time, Minutes 240 60 8 8 Product Recovery, Wt. percent 95 85 91 96 Product Distribution, Mole percent:

Xylene 22 28 25 25 Trimethylbenzene. 51 47 45 5O Tetramethylbenzene. 25 3O Pentamethylb enzene 0 5 0 0 Product Trimethylbenzene, Mole percent:

Pseudo cumene 65 70 Mesitylene 26 Hemimellitene 4 Product Tetramethylbenzene, Mole perce Durene 50 50 50 Prehnitene 5 0 Example D In this example, the effect of ethyltoluene in the feed to the process was examined. In Test No. 17, a mixture In Test No. 15, the results show that a system a of pseudo cumene, mesitylene and hemimellitene as the V teract with trimethylbenzene to form ethyltrimethylflj j Approximately 90% of the ethylbenzene and toluene. toluene disappeared. Tests 8 and 17 show that, if anything, the presence of ethyltoluene has a slight effect in TABLE V [100 vol. percent HF on feed-135 O. and 240 minutes] Test No 17 8 Feed Composition, Mole percent:

Pseudo cumene, 77% 100 Mesitylene, 6 o O Hemimellitene, 17%.. 0 Ethyltoluene 10 0 Product Recovery, wt. percen 96 Product Distribution, Mole percen Toluene 4 0 Xylene 13 19 Trimethylbenzene 61 62 Ethyltoluene 1 Tetramethylbenzene. 18 19 Ethyltrimethylbenzene 3 Higher Trace 0 Trimethylbenzene Gonverte Mole perce By disproportionation 35 38 By accepting ethyl groups... 4 Total 39 39 In the examples, the hydrocarbons have been separated from the HF catalyst by quenching the system with cold water. Since this procedure produces a dilute acid which is of no value for reuse without reprocessing, it is primarily a laboratory technique. At about 100 C. the trimethylbenzenes are soluble in liquid HP to the extent of about 15 volume percent so that 2 phases always exist in the contacting zone. The hydrocarbon phase may be separated from the acid phase by decantation or other physical separation means. When the conversion is being carried out so that the equilibrium mixture of tetramethylbenzenes is the product, the separated acid phase containing dissolved hydrocarbons may be recycled directly to the contacting zone. When it is desired to operate under conditions such that recycle of dissolved hydrocarbons is undesirable, the HF may be recovered in a substantially pure form by distilling or flashing the HF away from the higher boiling dissolved hydrocarbons. Owing to the very low boiling point of liquid HF, this separation may be carried out under pressure without cooling the liquid HF too greatly.

The above show that it is possible to produce a durenerich tetramethylbenzene fraction in good yield in an economic process involving relatively cheap liquid HF catalyst in relatively small amounts and at commercially feasible temperatures and times of contacting. Furthermore, they show that the process is applicable to trimethylbenzene feeds such as are readily obtainable from the petroleum industry.

Thus having described the invention, what is claimed is:

1. A process for the production of durene comprising (A) contacting, under substantially anhydrous conditions, a feed comprising essentially at least one trimethylbenzene, with liquid HF catalyst, in an amount between 25 and 100 volume percent on feed, at a temperature between about C. and C., for a time such that between about 20 and 45 mole percent of said trimethylbenzene is converted, said time being between about 2 minutes and 240 minutes, the higher temperatures and higher catalyst usage corresponding to the shorter times, (B) removing catalyst from a product hydrocarbon mixture comprising xylene, trimethylbenzene and tetramethylbenzene, (C) separating the tetramethylbenzene product, wherein durene is the predominant isomer, and (D) crystallizing essentially pure durene from said tetramethylbenzene pro-duct.

2. The process of claim 1 wherein said feed contains not more than about 10 mole percent of ethyltoluene.

3. The process of claim 1 wherein said catalyst usage 6. The process of claim 1 wherein the feed is a closeis between about and volume percent. boiling mixture of C9 aromatic hydrocarbons consisting 4. The process of claim 3 wherein the temperature is essentially of pseudo cumene, mesitylene, hemimellitene, between about C. and C., the mole percent and not more than about 10 mole percent ethyltoluenes. of trimethylbenzene converted is between about 20 and 5 30, and the time is between about 3 minutes and 60 References Cited in the file of this Pawnt minutes, the lower times corresponding to the higher U T STATES PATENTS temperatures.

5. The process of claim 1 wherein the durene-poor tetramethylbenzene fraction is cycled to the trimethyl- 1O benzene contacting step A.

2,416,184 Lee et a1. Feb. 18, 1947 

1. A PROCES FOR THE PRODUCTION OF DURENE COMPRISING (A) CONTACTING, UNDER SUBSTANTIALLY ANHYDROUS CONDITIONS, A FEED COMPRISING ESSENTIALLY AT LEAST ONE TRIMETHYLBENZENE, WITH LIQUID HF CATALYST, IN AN AMOUNT BETWEEN 2K AND 100 VOLUME PERCENT ON FEED, AT A TEMPERATURE BETWEEN ABOUT 130*C. AND 170*C., FOR A TIME SUCH THATAT BETWEEN ABOUT 20 AND 45 MOLE PERCENT OF SAID TRIMETHYLBENZENE IS CONVERTED, SAID TIME BEING BETWEEN ABOUT 2 MINUTES AND 240 MINUTES, THE HIGHER TEMPERATURES AND HIGHER CATALYST USAGE CORRESPONDING TO THE SHORTER TIMES, (B) REMOVING CATALYST FROM A PRODUCT HYDROCARBON MIXTURE COMPRISING XYLENE, TRIMETHYLBENZENE AND TETRAMETHYLBENZENE, (C) SEPARATING THE TETRAMETHYLBENZENE PRODUCT, WHEREIN DURENE IS THE PREDOMINANT ISOMER, AND (D) CRYSTALLIZING ESSENTIALLY PURE DURENE FROM SAID TETRAMETHYLBENZENE PRODUCT. 